In the field of mobile communications, technologies are proposed which enable handover between mobile communication systems with different frequencies or different radio access technologies (RATs). For example, when a user terminal communicates in a radio access network, the user terminal may perform handover with another radio access network to continue communications due to the propagation environment or the unbalanced network load. This in turn makes it possible to distribute the load of the radio access network to other radio access networks when the radio access network becomes congested, thereby increasing the total capacity and throughput of the radio access networks. In addition, when the coverage of the radio access network in which the user terminal is in communication is intermittently broken, the user terminal can perform handover with either another radio access network or the same radio access network with a different frequency to continue communications.
FIG. 1 shows handover between radio access networks with different frequencies. In this example, cells A and B using a first frequency f1 and a cell C using a second frequency f2 coexist in the same geographical area. When the user terminal (UE: user equipment) which is in communication in the cell C using the second frequency f2 moves toward the cell boundary of the cell C (the coverage end of the second frequency f2), the coverage of the second frequency f2 may be intermittently broken. In this case, the user terminal can perform handover with the cell B using the other frequency f1 to continue communications in the cell B and outside the coverage of the second frequency f2. Handover between radio access networks with different frequencies is described in 3GPP, TS 25.331 and TS 25.304, for example.